The present invention is directed to a radio frequency (RF) power supply for operating an electrodeless lamp, such as a fluorescent, molecular, or high intensity discharge electrodeless lamp. An RF power supply converts a DC voltage to a suitable radio frequency for the lamp and is typically part of the electronic ballast of the lamp. The RF power supply includes a ballasting inductor that is coupled to the electrodeless lamp to ignite and maintain the plasma in the lamp's discharge gas, without providing electrodes in the lamp bulb.
Because the complete electronic ballast includes numerous components in addition to the RF power supply (e.g., EMI filter, rectifier, PFC boost stage, DC bus electrolytic capacitors), the efficiency of the RF power supply is desirably 95% or more, which has not been achievable in a commercially available power supply. It has been found that one of the key factors in improving efficiency is reducing power loss in the ballasting inductor that is coupled to the lamp.
FIG. 1 shows a known circuit for an RF power supply whose efficiency is about 91.7%. DC power source E delivers a DC voltage to a pair of DC rails, with a electrolytic capacitor (parasitic inductance) C0. During operation, first inductor L1 is inductively coupled to lamp D. Transistors S1 and S2 are driven with a sinusoidal voltage (8-9Vp) delivered by driving transformer Dt that is tuned to a specific frequency (2.6 MHz) by capacitors CP, CG, and Ciss. Feedback capacitor Ci couples driving transformer Dt with the output voltage V1. Resonance capacitor CR is parallel to the first inductor L1 and coupling capacitor CC connects the output of the driving transformer Dt to one of the input terminals of first inductor L1 through the ballasting inductor LL. The resonant circuit is tuned on a frequency fRS (about 2.45 MHz) that is slightly lower than the resulting operation frequency (f0≈2.5 MHz). This RF power supply has a 13.5 W loss, of which 7.8 W are attributed to the ballasting inductor LL. This circuit is further explained in U.S. Pat. No. 5,962,987. The particular parameters for this circuit are shown in Table 1 (in FIG. 5) that includes operating characteristics for RF power supplies of the prior art (FIGS. 1-2) and of the present invention (FIGS. 3-4) for a same set of input parameters so that results can be easily compared.
FIG. 2 shows a variation of the circuit of FIG. 1 in which the voltage viewed by the half bridge (the voltage VG on CR) is reduced by inserting an additional capacitor CS in series with the first inductor L1, thereby avoiding the bulky coupling capacitor CC. This reduces the inductance of ballasting inductor LL and thereby reduces the losses in the ballasting inductor LL. The voltage drop on CS is VCS=I1XCS, which in this instance is about 190V. This reduces the viewed voltage VG on CR from 550V to 360V, which is a 35% reduction. This, in turn, reduces the inductance of ballasting inductor LL by 35% from 37 μH to 24 μH. The current in ballasting inductor LL can also be reduced from 3.8 App to 3.4 App by reducing the phase angle between IL and the fundamental sine wave V0f contained in the half bridge midpoint voltage, which is trapezoidal in consequence of ZVS. As a result, the loss in the ballasting inductor is reduced to about 4.4 W (with a further 3.6 W loss in transistors S1 and S2) so that the total loss is 9.4 W, thereby increasing the efficiency from 91.7 to 94.1%. This circuit is further explained in U.S. Pat. No. 5,446,350. The particular parameters for the circuit of FIG. 2 are also shown in Table 1.